Mollusk shell assemblages as archives of spatial structuring of benthic communities around subtropical islands

Abstract

Surficial assemblages of mollusk shells provide minimally invasive, quantitative data that are potentially adequate for assessing spatial organization of local benthic ecosystems. This study evaluates distribution and ecological characteristics of mollusk-dominated benthic communities around San Salvador Island (Bahamas). A total of 60 bulk samples collected along 12 onshore-offshore transects around the island yielded 20,301 specimens representing 180 mollusk species. Radiocarbon dating of individual shells (n = 30) indicated that these surficial accumulations represented a multi-millennial (time-averaged) record of local communities. Indirect multivariate ordinations (NMDS) separated mollusk death assemblage samples by locality, local habitat types, and regional context. Even in the case of comparable habitats within the same bay, samples clustered by transect location suggesting that surficial shell accumulations faithfully archive local differences in mollusk communities despite time-averaging and post-mortem shell transport. At the regional scale, a clear faunal separation is observed between windward and leeward sides of the island, suggesting that water energy represents an overriding regional driver that controls local composition of mollusk assemblages. Within each water energy regime, the faunal composition appeared to be primarily controlled by habitat type (seagrass versus sand habitats). Moreover, consistent with theoretical predictions that structured habitats enhance faunal stability of local communities, death assemblages from seagrass habitats were less variable in faunal composition than open sand habitats. Results indicate that San Salvador Island benthic communities are characterized by a predictable spatial organization controlled primarily by regional physical and, secondarily, biological processes. Also, the fact that extensively time-averaged shell assemblages have retained an interpretable spatial pattern suggests that regional physical processes, local habitat distribution, and structuring of mollusk associations must have been stable over multi-centennial time scales. This case study suggests that non-invasive sampling of dead mollusks can be an effective strategy for assessing prevalent long-term patterns in spatial distribution of regional faunal associations and examining processes that drive spatial structuring of marine communities.